1. Field of the Invention
This invention relates generally to semiconductor circuits composed of field effect transistors (hereinafter called "FETs"), and more particularly to semiconductor circuits which will be suitable for taking out a signal proportional to the threshold voltage of FETs formed in an integrated circuit (hereinafter called the "IC").
1. Description of the Prior Art
When a voltage above a threshold voltage V.sub.TH of an FET is applied to its gate, a current flows through its drain-source path and the FET can therefore be used for signal amplification or signal processing.
However, the threshold voltage V.sub.TH of FET fluctuates on an order of from 10 to the 200 mV due to fabrication process of IC. It also changes at a rate of about 1.5 mV/.degree.C. with the operation temperature of the IC. At this time, V.sub.TH changes by about 150 mV with a temperature difference of 100.degree. C. Furthermore, V.sub.TH of the FET changes wth the change of the gate length L.sub.g due to the short channel effect. If FETs each having a varying L.sub.g are used, V.sub.TH varies within the same wafer and moreover, the fluctuation of V.sub.TH, that is, .DELTA.V.sub.T, occurs for the fabrication fluctuation of the gate length, that is, .DELTA.L.sub.g, in the FET group having the same rating.
If V.sub.TH changes or fluctuates in the manner described above, transistor characteristics and the operating point fluctuate, too, so that circuit design becomes extremely difficult. In Si MOSFETs, efforts have been made to eliminate these adverse influences by taking into consideration various design margins. For example, "ISSCC, Digest of Technical Papers (1976)", pp. 54-55 describes an attempt which applies feedback to the potential of a well portion, in which a transistor is buried, and compresses .DELTA.V.sub.TH in order to restrict the fluctuation of V.sub.TH (corresponding to the change of .DELTA.V.sub.TH to .DELTA.V.sub.TH .noteq.0) when L.sub.g is around 2 .mu.m.
The drop of the circuit operation margin due to the fluctuation of the threshold voltage becomes a more critical problem from the aspect of circuit design because signal amplitude as well as power supply voltage must be reduced with a higher operation speed of the circuit and its lower power consumption. The necessity for eliminating the influences of the threshold voltage fluctuation .DELTA.V.sub.TH is particularly high in those circuits which use GaAs MESFETs. An attempt has been made conventionally to increase the margin of the circuit itself in order to cope with this problem, but no attempt has been made so far to stabilize the IC operation by use of a circuit for directly detecting the threshold voltage V.sub.TH or its fluctuation.